The present invention relates to non-contacting electrostatic voltmeters. Non-contacting electrostatic voltmeters measure an electrostatic potential on a surface without physical contact therewith. Typical non-contacting electrostatic voltmeters are described in U.S. Pat. No. 3,525,936, entitled "Electrostatic Voltage Follower Circuit for Use as a Voltmeter", issued to Robert E. Vosteen on Aug. 25, 1970, and in U.S. Pat. No. 3,611,127, entitled "Electrostatic Potential and Field Measurement Apparatus Having a Capacitor Detector with Feedback to Drive the Capacitor Detector to the Potential Being Measured", issued to Robert E. Vosteen on Oct. 5, 1971. These voltmeters have mechanical devices for varying the capacitance between a detector and the surface. This varying capacitance produces an alternating current error signal on the detector proportional to the difference between the DC voltage of the detector and the potential on the surface. A feedback circuit is coupled to the detector for causing the DC voltage of the detector to be adjusted to a value equal to the potential of the surface in response to the error signal. The potential on the surface is determined by measuring the DC voltage on the detector. However, the mechanical devices required to vary the detector-surface capacitance are undesirably complex. Further, in order to adjust the DC detector voltage, an undesirably expensive, adjustable high-voltage power supply is required.
Other typical non-contacting electrostatic voltmeters have detectors capacitively coupled to the surface and to ground. The potential of the surface can be determined from the DC voltage of the detector and the values of the detector-to-surface and the detector-to-ground capacitances. However, it is undesirably difficult to accurately determine the detector-to-surface capacitance. Without accurately determining this capacitance the potential of the surface cannot be precisely measured with this type of voltmeter.